Today we talk about the “DEVELOPMENT
OF MICROSTRUCTURES IN IRON–CARBON ALLOYS”.
Phase changes that occur upon passing from the ϒ region into the ∝ + Fe3C phase field are relatively complex.
Consider,
for example, an alloy of eutectoid composition (0.76 wt% C) as it is cooled
from a temperature within the ϒ phase region, say, 800˚C, that is, beginning at point a and moving down the
vertical line xx’. Initially, the alloy is composed entirely of the
austenite phase having a composition of 0.76 wt% C and corresponding
microstructure. As the alloy is cooled, there will occur no changes until the eutectoid
temperature (727˚C) is
reached.
The
microstructure for this eutectoid steel that is slowly cooled through the eutectoid
temperature consists of alternating layers or lamellae of the two phases (∝ and Fe3C) that form
simultaneously during the transformation. In this case, the relative layer
thickness is approximately 8 to 1. This microstructure point b, is
called pearlite because it has the appearance of mother of pearl when
viewed under the microscope at low magnifications.
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